1. Field of the Invention
The present invention relates to a fluid dynamic pressure bearing, a spindle motor including this fluid dynamic pressure bearing, and a recording disk drive including this spindle motor.
2. Description of the Related Art
Conventionally, a bearing of a motor that drives a recording disk employed in a hard disk drive, a removable disk drive or the like has been variously proposed. As shown in FIG. 10, for example, fluid dynamic pressure bearings 512 and 514 using a dynamic pressure generated from a lubricating fluid such as an oil held in micro gaps between a shaft 502 and a sleeve 504 during rotation of the motor are employed as bearings of the motor.
For the fluid dynamic pressure bearings 512 and 514, a pair of dynamic pressure-generating grooves 506 and 508 are formed on an inner peripheral surface of the sleeve 504. These dynamic pressure-generating grooves 506 and 508 are herringbone-shaped, spiral or the like. The paired dynamic pressure-generating grooves 506 and 508 are adjacent to each other and constituted by alternately arranged hills and grooves, respectively. A difference in height between the hill and the groove corresponds to a depth of each dynamic pressure groove.
As a method for forming these dynamic pressure-generating grooves 506 and 508, press working is often selected. The press working is a method for pressing a hard cylindrical pin having a groove pattern formed on its outer periphery against an inner peripheral surface of the sleeve 504, and for transferring the groove pattern onto the inner peripheral surface of the sleeve 504. Since the press working enables simultaneously forming the paired dynamic pressure-generating grooves 506 and 508 on the inner peripheral surface of the sleeve 504, the sleeve 504 can be manufactured at a low cost, as compared with other machining methods such as electrochemical machining and rolling.
FIG. 11 is an enlarged cross-sectional view that depicts important parts of the spindle motor shown in FIG. 10. FIG. 11 is an enlarged view that particularly depicts the inner peripheral surface of the sleeve 504 on which the dynamic pressure-generating grooves 506 and 508 are formed. An annular groove 510 is formed between the dynamic pressure-generating grooves 506 and 508 on the inner peripheral surface of this sleeve 504. By forming the annular groove 510, a radial gap between the annular groove 510 and a shaft 502 facing the annular groove 510 is enlarged. It is thereby possible to prevent an increase in a bearing loss resulting from a current rise following an increase in an oil internal pressure of the radial gap while the motor is driven.
However, if such an annular groove 510 is to be formed simultaneously with formation of the dynamic pressure-generating grooves 506 and 508, the following disadvantages occur. As shown in FIG. 11, if a protrusion of a cylindrical pin corresponding to the annular groove 510 is pressed against the inner peripheral surface of the sleeve 504, surrounding regions of a region in which the annular groove 510 is to be formed are also attracted toward a direction in which the protrusion of the cylindrical pin goes forward (downward in FIG. 11). As a result, hills 508a and 506a adjacent to the annular groove 510 are formed lower than the other hills by At. Due to this, while the motor is driven, boost pressures applied to central portions of the respective fluid dynamic pressure bearings 512 and 514 cannot be increased in the regions of the dynamic pressure-generating grooves 506 and 508 adjacent to the annular groove 510, respectively. Predetermined pressures cannot be obtained in the respective fluid dynamic pressure bearings 512 and 514, accordingly. As a result, rigidities of the bearings 512 and 514 are reduced and the bearings 512 and 514 are often incapable of stably supporting the shaft 502.